1. Field of the Invention
This invention relates to a copying apparatus of the slit exposure type in which a flat stationary original is scanned by a rotatable scanning mirror to thereby exposure a photosensitive medium to the image of the original at high speed and without swing and tilting of the image.
2. Description of the Prior Art
It is well-known that the exposure scanning system using a rotatable scanning mirror is highly effective in terms of vibration and mechanism to realize high-speed copying. FIG. 1 of the accompanying drawings in a schematic view showing an example of the conventional exposure scanning system using a rotatable scanning mirror. A flat original 1 is scanned as a slit portion original 3 having a width lo by a rotatable mirror 2 and this slit portion original 3 is imaged on a projection image plane (a photosensitive medium surface) 6 by a projection optical system 5. However, when a flat original is scanned by the use of the rotatable mirror 2, there is a difficult problem that the length of the light path between the original surface 1 and the photosensitive medium surface 6 is varied with the rotation of the rotatable scanning mirror 2. That is, if the position of the rotatable scanning mirror when the distance between the rotatable scanning mirror 2 and the original surface is shortest (Lo) is the standard position and the distance between the scanning mirror 2 and the original surface 1 when the scanning mirror has been rotated by .theta. from the standard position is Lo+.DELTA.L, the following expression can be indicated: ##EQU1## where .DELTA.L is the amount of deviation of the length of the light path caused by the rotation of the scanning mirror 2. In the usually thinkable condition of use wherein .vertline..theta..vertline..ltoreq.25.degree. and Lo=400 mm, .DELTA.L reaches a maximum of 41.4 mm.
To correct such variation in the length of the light path, it is disclosed in Japanese Patent Publication No. 9182/1975 that parallel-movable mirrors are provided before and behind the projection optical system leading from the rotatable mirror to the image plane to correct the variation in the length of the light path and that the projection optical system is moved to eliminate any fluctuation of the conjugate arrangement. Also, U.S. Pat. No. 3,537,373 discloses that two lens elements within the projection optical system are synchronously operated parallel to the optic axis to thereby eliminate any variation in the length of the light path and any fluctuation of the conjugate arrangement.
However, it has been found that the use of a rotatable mirror in a copying apparatus raises another difficult problem in addition to the variation in the length of the light path. It is a problem that the original 1 falls in accordance with the angle of rotation of the rotatable mirror 2, as viewed from the photosensitive medium surface 6 side. That is, if a mirror image 4 of the slit portion original 3 by the rotatable mirror 2 is considered, the mirror image 4 is inclined by the angle of rotation .theta. of the mirror 2 with respect to the optic axis of the projection optical system 5. (This state will hereinafter be referred to as the swing and tilting.) The image 7 of the slit portion original 3 on the photosensitive medium surface 6 by the projection optical system 5 is also made to swing and tilt by .theta.. This phenomenon is such that where the copying apparatus is used with the slit width lo of 10 mm and with the effective F-number of the copying optical system being 10, the out-of-focus width .DELTA.l' of the slit image 7 on the photosensitive medium 6 is as great as 0.2 mm in the case of .theta.=25.degree., as shown in FIG. 2 of the accompanying drawings, and such out-of-focus width is practically impermissible.